Automatic trunk selecting system for radio telephone stations



Dec. 12, 1950 H B. BARTELINK 2,533,662

E. AUTOMATIC TRUNK SELECTING SYSTEM FOR RADIO TELEPHONE STATIONS Filed Anril m, 1947 e kcdu:

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HARM-GEN Map INVENTOR.

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AUTOMATIC TRUNK SELECTING SYSTEM FOR RADIO TELEPHONE STATIONS Everhard H. B. Bartelink, Bronxville, N. Y., as-

signor, by mesne assignments, to Automatic Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware Application April 11, 1947, Serial No. 740,959

9 Claims.

This invention pertains to two-way radio communication systems, and, more particularly, to multi-channel radio telephone systems comprisinga central oiiice and subscribers stations linked thereto by radio.

A primary object of the invention i to provide means whereby the central office may call any of the subscribers stations, or vice versa, over any of a number of radio channels operating on different frequencies.

In this connection, it is well known from wire telephony that for a given number of subscribers stations and a given number of trunks available to establish connections between them, the traffichandling capacity of the system is much larger if all subscribers have access to all trunks than if the subscribers are subdivided into groups, each group having access to only one trunk. In order to secure this same increase in traffic-handling capacity in radio-telephone systems of the character above noted, it is proposed, in accordance with the invention herein, to give all subscribers to this service access to more than one radio channel for initiating calls to or receiving calls from the radio-telephone central oiiice. particularly important in view of the extremely small number of radio channels allocated to the common carriers.

In a multi-channel radio-telephone system, it is a simple matter for the central oifice operator to select an idle channel on outgoing calls, since her switchboard lamps indicate which channels are busy'and which are idle. From the subscribers end, however, the problem is not so simple, as he has no way of foretelling when and on which channel a call might be incoming. Conversely, on calls outgoing from a subscribers station, the push button method of idle radio channel selection is objectionable, because it necessitates testing until a free channel is found.

These difficulties and objections are overcome, in accordance with the present invention, by providing each subscriber's station with a mechanism for continuously and progressively testing all :radio channels for incoming calls. Upon receipt of an incoming call, the testin mechanism, upon contacting the channel on which the call is placed, holds the subscribers station connected to that channel until it is determined whether or not the call is intended for that particular station. If not, the testing is automatically resumed, but otherwise the connection is held and the subscribers station signaled. On calls outgoing from a subscribers station, the testing mechanism opcra es in a generally similar fashion. That is to This is .4:

say, upon removal of the subscribers receiver from the switch hook, the testing mechanism continues to operate until an idle channel is located, whereupon the mechanism holds the calling station connected to that channel and signals the subscriber to that effect.

Although the invention is applicable to radiotelephone systems of the non-automatic type, employing, for example, code ringing or equivalent means for signaling the particular subscribers station called, it is preferably employed, as disclosed in detail in its preferred embodiment herein, in automatic telephone systems employing dialin pulses for station selection, the subscribers stations being equipped, to this end, withsuitable station selector mechanisms responsive to incoming dial pulses.

Thus, in accordance with the preferred embodiment of the invention, each subscriber's station comprises radio transmitter and receiver elements, each having associated therewith a multiplicity of fixed tuning elements, such as piezoelectric crystals for tuning the same to the frequencies of the variou radio channels respectively. A motor-driven rotary switch and associated relays are provided in conjunction with said tunin elements for progressively, momentarily and repeatedly tuning the transmitter and re:- ceiver to each of the radio-channel frequencies aforesaid so long as the system remains idle. When the central office operator desire to place a call, she selects an idle channel and transmits carrier frequency of that channel, modulated by dial tone, to all subscribers stations. As the motor-driven rotary switches at the various idle subscribers stations arrive at their respective switch contacts, which tune the radio receivers thereat to the frequency of the received carrier, the dial tone actuates the associated station selectors in such manner as to stop the motors and thus hold these stations tuned to the received carrier, so long as dialing tone is impressed thereon. The dial tone is thus transmitted for a sufficient inter- .val before dialing is initiated, to assure that the rotary switches at all idle stations will have opportunity to contact the channel on which the dial-tone modulated carrier is received and to stop the motor rotation thereat.

The operator then proceeds to dial the station desired. The station selectors at all idle stations are actuated by these dialing pulses. At the conclusion of the dialing, the station selectors at all stations other than the particular station called, are restored to normal, thereby to energize the motor driven rotary switches to resume test! 3 ing for incoming calls. At the particular station called, however, the rotary switch is held inoperative until the call is completed under control of the subscribers switch hook and of the incoming carrier.

To initiatean outgoing call, the subscriber liemoves his receiver from the hook and presses an outgoing call button. This prepares a circuit for deenergizing the motor and arresting the rotary switch as soon as it contacts a radio channei on which no carrier is incoming from the central ofiice, this condition being indicated to the sucscriber by means of a busy tone or'visual signal. The motor may thereafter be helddeenergized under control of the subscrib efsswitch hook, or, alternatively, under control of the subscribers switch hook and a carrier current actuated relay The subscriber.

in the radio receiver output. thereupon depresses his handset pushoutton to switch on outgoing carrier of his radio transm tter, thereby to signal the central office of-his e to place a ca11."*r"ne operatonupon answerswitche's" on"carrier incoming to the subscr1beirsstation 'on the channel induestion, thus it stamina bennecnbn in me manner" above 'described for incoming calls, or alternatively, under exclusive, oln'trofo f the su s riber-e switch hook.

h d g l is' ra ses diagram of a subscribers stamont ge rd (as with oneinod'ification of the ve outlined, according to which ta y'swlt Kmtolisfihderjoint control of w {yateirheoranabr the carrier on oingas'well as onincoming calls; 2 1s ar semenary circuit detail illustrating modification in which the rotary switch motor ispcontrolled exclusiveiy' through contacts of isiswitch'hook on outgoing calls answer s t Ei t eir is hown a n sa as, sw rheier d e rad e 9f conve a desi n ed meme a ni'aste seesaw ll, working into an harmonic or f i l i'n'e arriwfi i ie r d oth? i an see teq' er "me nest im ar y 5 W? er sar i were eaan" mo i ne ato a d we fd he iiie a. power 'Thfereeeiver Bis provided with a. u r i in up adi i na s in thecent l'oificeto the subscriber's i; 2 .1- The. re smiii f i iii arl n r wi ianantenna l8 1 for transmitting outgoing s als thereto. or tuni re 'the radio receiver R to the frequencies' frl fr' 2 'fr'-3,' etc. of 'the rad-i genesi there are 'provided a series of fixed nemesis, such as "piezoelectric crystals C, et "individually connectiblefrom ground ugh i 'fa ei ir I I Y T sooiat ed' front contacts of the normally relays D, E', new, to the control gr of 'thernaster oscillator when, the cathorie'oi 'whi ch is grounded at 'Zil, through the plate current biasing esistor 24.

For similarly radio transmitter T s e enews 1275?; 1. i efr i Q e radiofchanne ls, crystals a, b, c, etc., are conblethr he ai'mainlf -W 9 its; an 159 s te ni'p eqis h o mallyfdeenergized, transmitting frequency con rel s etcl, between'grid' and cathode thetranim ter master oscillator tukie "at, the

a; tease an also on incoming cja l ls aiter'the subscriber 4 oscillator circuit of which is similar to that of the receiver master oscillator R above described.

The associated pairs of receiving and transmitting frequency control relays Dd, Ee, Ff, etc., are connected in parallel between negatively grounded patterytl and over leads 3 g 3q, 34, etc. to successive stationary contacts, suchas 35, of a rotary switch RS having a grounded rotary contact arm 38 adapted to sweep across the stationary contacts 35 successively. Switch arm 36 is ;eyed to a shaft 3'! of a normally energized, electrical motor M, whereby the pairs of frequency control relays Dd, Ee, Ff, etc. are successively, momentarily and repeatedly energized to tune] the radio transmitter and receiver T, R to the carrier frequencies fr-l, fr2, fr3, etc., of the various radio channels.

a pair of group receiving and transmitting rela'ysGR and GT have energizing windings G, H, 1, etc. and g, h, 2', etc, which are connected from negatively grounded batteries 46 and 4-1 to the leads3 2f33, 34, etc. respectively. Thus, as rotaryswitcharm '36 passes over contacts 35, the GR and GT relays'will repeatedly energize and release, along with the frequency control re r e see; s

hen, he group ec i in re ay, GR. is neram s. noe an t co nects t e ardl s einequency output terminal 50 of the radio receiver, thmush the R-l a mat e. e t: onta Of e GR r ay and ever eedi ct rs 5i tc h e eiifjice e r r 4 Qfib s re 11 we; 5 and also ever ude 51 to the tuned, dial t r ctifie irc li 'i q iii? uh9 befs saw der QW'Il emer 51.56;-

GB el als functions rr iza hrq hg ie r. l. tie e di r uit for he; r s e weird e a n Hammin el s thereb t ma n ain hemeb ilener izai fo th dura n i ae an; and q' pre are t re e s? 1: arma e nd rent conta n s were; ig. and, t mr rarr h q rwu e the MC e a n ou o n alls eseipla d he inaft r ni s co ne tion he u o n cell and: re a c nt l res tsare shown generally at 591 i T e exam ransm tt n relay GI-r un iitn w when energized, to connect t he subscriber's handsa'transnaasr'su over connection 61'; the front; Contact and GT l armature of the GT relaywto the "radio transmitter "modulator inputliz Iljl addition, the GT relay prepares a circuit traced throughits armature andassociated front contact for switching on the transmitter carrier, asexp j lined below? The motor M 'whi'ch drives the rota-17y switch R8 is normally energized from battery GZ thr-ough the MC farmature and back contact of 'the normally deenergized; motor controlrelay- MC, which, howeverfwhen energized, short-circuits" the motor armature over connection fi3'extend' guesses tem, the remaining circuits and operation can now best be explained first with reference to incoming calls and then with reference to outgoing calls. a

" Incoming calls At the central ofiice, the operator will, on placing aoall to one of the subscriber's stations, first select an idle radio channel, plug in her cord to switch; on the outgoing carrier, and apply dial tone. This dial tone will be applied for a sufiicient interval to permit the motors M at the various subscribers stations to make at least one complete searching sequence before dialing is ini- .tiated'.)

Now reverting to the subscribers station as .showninthe drawing, the dial-tone modulated carrier will be picked up by the receiving antenna l'l. Let us assume,,at this point, that the .operator,has selected the radio channel corresponding to frequency ir 2, and thatat the in- 6, MC relay thus stops motorM with the {rotary switch arm 36 resting on the contact connected to lead 33. Whereas the P'relay is fast-operat ing and responds to dialing pulse interruptions of the dial tone, the SR relay, as stated, is slowreleasing and hence remains operated during dial-=- ing. Hence, the SR relay will remain operated to hold the MC relay energized and the motor M deenergized so long as dial tone is received.

As above stated, the operator applies dial tone for a sufficient period before dialing'to permit the rotary switches RS'at all idle subscribers stations to come to rest on the contact which tunes the station to the frequency of the incoming carrier, whereupon the rotation of all switches is ar rested for the duration of the dialing tone in the manner aforesaid. When sufficient time has elapsed as measured at the transmitter, a start signal is given the operator, whereupon she dials the number of the station desired to be called. During this period, the motors M-at all stations are stopped as aforesaid and the station selectors are registering the call in the manner now to be stant the carrier is first picked up by the receiving .zantenna, the rotary switch arm 36 is on some contact other than that connected to lead 33. Assume, for example, that at the instant in question, it is on the contact connected to lead 34.

As the switch arm 36 driven by motor M sweeps over contacts 35, it will, while on each contact, -momentarily energize the transmitting and receiving frequency control relays, such as Ff, to .tune the transmitter and receiver to the corresponding frequency, and will also momentarily energize theassociated GR and GT relays, the

former to connect the radio receiver output to the station selector 51, 58. Since, however, under theabove assumption, dial tone is being transmittedonly on carrier frequency jr-2, nothing 'will occur to arrest the motor rotation until 'switch arm 36 sweeps onto the contact connected to lead 33.

At the instant it does this, however, the Ee relays will operate to tune the transmitter and receiver to the frequency fr2 of the dial-tone modulated, incoming carrier. And concurrently therewith, the GR and GT relays will operate, so that'the dial tone will be transmitted from the receiver output 50, through GR-l and its front 'contact, and over connection 5|, 52, 56 to the tuned station selector amplifier and rectifier ciricuit 51, the rectified output of which traverses the pulse relay P of the station selector. This, in turn, will energize the slow release relay SR (see circuit 58) of the station selector, over a cir- Icuit traced from negatively grounded battery 38 throu h the SR relay winding and thence to tground at 69' through the front contact and P-| armature of the P relay. The SR relay, upon operating, will in turn complete a circuit to energize the motor control relay MC, this circuit being ,traced from negatively grounded battery 6! 1" through the back contact and OC.-2 armature of the normally deenergized outgoing call relay 0C, and thence through the front contact and SR'-'3 armature of the now energized SR relay, to ground at 10; The resulting operation of the" described.

The station selector shown at 51, 58 is similar to that described in my copending application Ser. No. 729,828, filed February 20, 1947, now abandoned. Hence, its circuit and operation will be but briefly touched upon herein. In addition to the P and SR relays, the selector includes: a

pulse repeat relay PR which is normally energized from battery 68 to ground at 69 through the P I armature and back contact of the P relay; a two-bank, minor selector switch SS including a stepping magnet SM for stepping the grounded rotary switch arms Hi, H to successive contacts 12, E3 respectively, a release magnet RM for restoring the switch arms to zero, and a decade switch DS for transferring the code switching from the lower bank of contacts 12 to the upper bank of contacts 13. In addition, the station selector includes a release control relay RC and a calling or bell ringing relay BR.

As explained in my copending application referred to, during dialing, the stepping magnet SM will be successively actuated by the PRI relay in response to dialing pulse actuations of the P relay, the SR relay remaining energized. As each digit of the called number is dialed, the selector switch arms 10, ll will thus be stepped around correspondingly. However, at all stations at which the switch arms 10, ll come to rest at'any stage of the dialing on contacts l2, 13, other than those to which the RC, DS and BR relays are wired, as shown, the release magnet RM will operate to restore the switch arms to zero. And

'since these contacts are wired in accordance with preselected codes which are different for each station, only at the station called will one of the switch arms I0, H always come to rest on a contact wired as aforesaid, in which event the RM magnet is prevented from operating except upon the contact to which the DS relay is wired, the RM magnet being otherwise held from operating by the SR and RC relays which remain energized,

whereby ultimately the BR relay is energized and locksupto ground at 1,5through its BRrayi rumatri an a s t r n ontact .en

through agbaok contact and fI'B--l armature of .ahorinally, deenergized transmitter button relay gize the associated motors M and RS switches, -thus resume searching for incomingcalls. .Atthecalled station, however, the MC motor is ,held deenergizedaiter cessation of the dial .tone

- by the holding circuit providedby the frontconand ,BR--I armature of the BR relay conheated in parallel to the SR-tarmature and frontcontact of the SR relay.

During dialing, the DS relayat thecalled sta- .-tion.-isenergizcd from battery '55 when switch arm comes to rest on contact '11, therebyconnectlug, ground to the relay winding. The DS relay, vuponoperating, locks up temporarilyto ground at l8 through its DS-2 armature and front contact -.and,the SR-Z armature and front contact of the SR relay, which, as stated, remains-energized ..,during reception'of dialing tone. Thus, theDS relay releasesas soon as the dial tone iscut off.

.Now, when the BRrelay is actuated at the called station as aforesaid, it lights a lamp 18a inrshunt therewitnand also completes an additional circuit for actuatinga bellor buzzer 16b, from batteryilflc to ground atlB through the front contact..andBR--.3 armature and front contact of .the BRirelay and thence through the front contact and SR/+2 armature of the SR relay. This hell or buzzer will accordingly remain energized eitheru'ntil the dial tone is cut off .to release the ,SR relay, or alternatively until the called station answers and depresses a push-button it provided on the handset 55, thereby to connect ground at 80 to the'TB relay winding, which receives bat- .teryat 8|. The TB relay, upon operatingopens .at' its 1313-4 armature and back contact the holding circuit for theBR relay, which thereupon releases to open, at its BR3,armature and front scant-act, the aforesaid energizing circuit for the }bell or buzzer 581;, which is thereupon deactivated, dialtone having been taken off the line bythis timeto open the parallel holding circuit traced ,,th rough the. SR- 2 armature of the SR re ay. ;C8 SS2l{ti0Il of the dial tone prior to answer by the subscriber does not release thejBR. relay or ex tinguish lamp 78a. These remain energized until the subscriber answers and actuates thepush- .button' lather-shy to energize the TB relay.

.Sinceat'the called station,.theMC relay was initiallyenergized through the SR'-.3 contacts and then held through contacts ,BR.-l.upon rele ase of ,SR'3, a third holding circuit is required for this relay for the duration of the call upon release of the BR relay when the subscriber. an-

swers. This third holding circuit is traced froni battery 61 andthe MC relay winding, throughthe normally open, but now closed, contaetgBZ-ot' the .subscribers handset switch .hook SH, thence through the OC!. armature and backrcontact, of

,ihe normally .deenergized OC relay, through the front contact and.GR-2 armature :of-the GR rea .66 E W through .the winding of the, outgoing 8 relayCRds connected, as shown in the output stage of the radio ,recivehhand lseneijgizedtby arectifiedcon ponent of the received carrier ouri+ rent. This relay is accordiuglyenxergimd'hy D" coining carrier-of the. frequency, tol .which;,the,re'-

ceiveipis; tuned and, remains, thus .en e rgized untll it isswitched oil. ZF-hus, inthe present instanlce, the Mc -relay. will be energized l at,the .called sta tion for the duration of the call, ,ThelVlC 1;, 2y

is provided witha short-cirouitingwinding closed through MC- 5, whereby it is ,iast to{,opratdbut slowto release. y f; I

As above explained, operation .of the,GB. and GT relays at the called station connects the subscribers handsetreceiyer 54 to..the radio receiver output, and connects the handset transmitter 60 to the radio transmitter input,and.also prepares a circuit 'for switching on theoutgoingccarrier.

This latter circuit is completed when thesubmoved and tube- 30 then osoillates-at-the mquency determined by the crystals at, b,c,et'c. cohnected between its grid and cathode, in the present instance, crystal'b co rresponding'to outgoing v carrier frequency fr2.

Atgthe conclusion of the calL-"theMC relay will be deenergized either by the subscriber replacing his'handset on the switch hook SH to open at contact 82 the holding circuit for the MG relay, or by the central oilice operator switching off the incoming carrier to releasethe'CRf-relay and thus open atits CR--l contacts,'the'MC-relay holding circuit. In this way, searching 0f th'e rotary switch RS is resumed at the calle'd station, whereupon the GR and GT relays. releasealong with the previously energized frequencycontrol relays, in this case Be. The system is thusrestored to" its normal or idle condition; slnce"; all

other relays were previously ,res'tored as-"expiained.

Q .outgoinocalls .0 euteeins h sub ib W new the handset from [the switch hook. SH auditiopress theputgoing call button, PB, in theoutgoing call circuit 59. .This .will energi ze theolltgoing callrelay QC, providednorinoOm l -fiaill i been registered, so that the BR r lfly is deenergized, ana m mec further that, no diahtondds being cei ed, $0. that the fi Rrelay i deenergized, .this;,,circuit; being ,traced iromrb r M thence through the contacts, of 'tli e 'hutgo s bu o B n eth ous h back we" t n sn-2 armature-ofthe sa en s ce through thegback contact and BRA-4 arrnat,

, theBR relayto ground at,,85. ,If VB. tla s are e e i i d t ame .hqld true. I5 the ,MC; relay since, as .was, explainediin connecvho thiinpom ns cel ther operet ns e dhfi 1 8. ci c its for the 7M relayare KIZQ CL-IWOBME the back contacts and SR-3 and BR-l armatures of the SR and BR relays.

'MC3 armature and back contact of the MC .relay to ground at 85. During this operation, no

outgoing carrier is being transmitted from the subscribers station, inasmuch as the control circuit for the outgoing carrier, as above traced in connection with incoming calls, includes the MC-d armature and associated front contact of the MC relay. In this connection, the circuits ,which initially energize and hold the MC relay .on incoming calls are broken at the OC2 armature contacts of the now energized C relay. However, when the 0C relay operates and locks up as aforesaidon outgoing calls, a new circuit is prepared for operating the MC relay, this cir-- cuit being traced from battery 61 through the MC relay winding, thence through the now closed contact 82 on the switch hook SH, thence through the OC-l armature and front contact of the now energized 0C relay, to the front contact of armature GR-3 of the GR relay, and thence through the back contact and grounded armature CR-l of the CR relay to ground at 2!]. This circuit will energize the MC relay as soon as the rotary switch RS tunes the radio receiver to a radio, channel on which no carrier is incoming from the central office. When such a channel is located, the GR relay will be operated as aforesaid, but the CR relay will be released, the latter due to the absence of incoming carrier, thereby to complete the operating circuit for the MC re- .lay above traced. When the MC relay is thus operated, it switches the aforesaid hold ng circuit for the 0C relay from ground provided at 85 throughthe MC3 armature and back contact of the MC relay, to the ground provided at 20 through the MC3 armature and front contact of the MC relay, over connection 86, and through the GR-3 contacts of the GR relay when energized and the CRI contacts of the CR relay when deenergized. In this position, the MC relay will hold, so long as no carrier is incoming from the central office on the radio channel in question, thereby to hold the CR relay inoperated. So long as the motor is running and the rotaryswitch thereby searching for a radio channel on which no carrier is incoming, the green light 64 will remain lit to so advise the calling subscriber. When, however, the motor is deenergized, as above ex lained. to bring the rotary switchto rest on an idle channel, the green light will be extinguished and the red light 66 energized by operation of the MC relay, thus advising'the calling subscriber that he may now actuate the handset button 19 to switch on the outgoing carier from transmitter T. In addition, the subscriber will receive a busy tone from source 19a through the MC5 armature and back contact of theMC relay until the relay operates. At this. point, it is to be noted that upon actuation of the MC relay, the transmitter power control circuit is closed except at the TB2 contacts of the TB relay,'inasmuch as the MC4 contacts are closed by operation of the MC relay, and the GT-2 contacts are closed by operation of the GT relay; Accordingly, when the handset push-button I9 is depressed by the subscriber, the TB relay will operate the TB2 contacts of the power circuit, thereby to switch on the outgoing carrier. The carrier thus outgoing from the transmitter T will be picked up by the central office receiver on the idle channel, and thus signal the operator, who in turn will switch on the carrier outgoing from the central ofiice and incoming to radio receiver R at the subscribers station. This, in turn, will operate the CR relay to transfer its CR| armature from its back to its front contact. This releases vthe 0C relay, thereby reestablishing the same holding circuit for the MC relay as is used on incoming calls, namely, the holding circuit traced from ground at 2|] through the CR! armature and front contact, the GR2 armature and front contact, the back contact and OCI armature, inasmuch as the above traced holding circuit for the 0C relay has now been broken at the back contact of the CRI armature, and has accordingly released, the circuit being completed thence from the 0-0-1 armature through the switch hook SH and its contact 82 to the winding of the MC relay. It is to be noted in connection with the above that the 00-4 and MC-3 armature contacts are of the make- -before--break type, so that the old circuit is not o of the carrier relay. This starts the motor and rotary switches searching again to release the group a sm tting and receiving relays GR and GT, and also the operated frequency control transmitting and receiving relays Dd, Ee, Ff, etc, whichever pair had been operated.

qroup relays and frequency control relays, s1m11ar to GR, GT and Dd, Ee, Ff, etc., may be provided for other radio channels and connected to contacts 35 of the rotary switch RS over conductors, such as 90. Also, the inputs to and outputs from additional transmitters and receivers may be multiplied to conductors 9!, in the same manner as the subscribers stationshown in the drawing. In this'way, a sequential search can be performed throughout a multitude of carrier frequencies, which can be located in wholly differ ent frequency bands, and can be received and transmitted over different receivers and transmitters, and can also serve different subscribers stations.

For a given radio channel, the carrier frequency incoming to the subscribers radio receiver R may of course be the same or different from that outgoing on his radio transmitter T.

It will be observed that the system above described provides a holding circuit for the MC relay at the CR-I contacts of the CR relay after a connection has been established. In the case of a mobile subscribers station, this might result in a disconnect during a call due to release of the CR relay, should the mobile unit pass through an underpass or other dead spot.

In such case, the Fig."2 modification may be employed wherein once a connection is estab lished and the subscriber's handset removed from the switch hook, the MC relay is held deenergized exclusively through the switch hook contacts. In the circuit of Fig. 2,. the push-button switch PB and the 0C relay are omitted. To E t-fill o tgoing calls, the subscriber removes his any said. carrierat 11 handset fromtthe switch hook: This completes a circuit to operate the MC relay as soon as the rotary switch RS connects the subscribers station to "aradioichannel on whichno'carrier is incoming. This circuit is traced from negatively grounded battery'fi'l through the winding of relay MC, the switch hook andassociated contact 82,

the MC-3 armature and back contact of the MC 1 relay, the front contact and GPWS armature of the GR relay and the back contact and grounded CR-| armature of the CR relay. As soon as the rotary switch RS passes onto contacts of an idle channel, this circuit willbe closed to actuate the MC relay, which thereupon locks up under con trol of the switch hook through its MC-A armature and grounded front contact; The MC relay .will accordingly remain energized to hold the rotary switch BS in the channel in question until the subscriber replaces his handset on the hook, thereby to break this holding circuit at SH-'i-83. As above stated, the MC'3 contacts of the MC relay are of the make-before-break type. The-subscriber will be advised as to when an idle channel hastbeen seized by cessation of the busy tone 19m.

On incoming calls, the circuit of Fig. 1, as modified'according to Fig. 2, remains unchanged in. its operation as above described, inasmuch as thesubscribers handset is on the switch hook until the called station'answers.

Iclaim:

1. In a radio receiving system selectively responsive to modulating signals received thereby on'any of aplurality of carrier current frequencies: a-radio receiver, a plurality of tuning elements for tuning said receiver to said carrier frequencies respectively, a motor and switching means driven thereby to'connect said tuning elements tosaid receiver'successively, a normally closed circuit for continuously operating said motor, a tuned circuit and'relay means connected thereto, responsive to a modulating signal received on any of said carrier frequencies for deenergizing said motor with said receiver tuned tosaid carrier, and meansresponsive to energization of said relay means for connecting a station selector mechanism to-lsaid tuned circuit, said selector mechanism responding'to coded mod ulating signals received on said: carrier for signaling at said station. and for completing a circuit for holding said motor'inoperative' upon cessation of said modulatingxsignal.

2. In a multi-channel radio communication system: acentral'office equipped. with a plurality of radio transmitter and radio receiver channels operating on different carrier frequencies, a pin-- rality'of remote stations each equipped'with'a radio transmitter and radio receiver, a plurality of tuning elements at each station, a motor and switching means driven thereby for connecting said tuning elements in pairs to the'transmitter and receiver thereat for concurrently tuning the same to said carrier frequencies successively, a normally closed circuit forrcontinuously operating said motor, a selective circuit and relay meansconnected thereto, at ieachstationand responsive to a modulatingfrequency impressed on said" centralstation for stopping the motors at all saidv stations with the transmitters and receivers thereat tuned tosaid carrier, a station selector oircuitat each station thereafter responding to codedinterruptions of said modulating frequency for selecting and signaling any said remote station, and means. in cluding a manually actuated switch at' each Sta:

tion for causing said motor to connect said'sta tion to an idle radio channel on outgoing'calls.

3. In aradio-telephone system: asubscribers station, including a radio-telephone transmitter and receiver; aplurality of tuning elements for tuning said transmitter and receiver to any of-a preselected number of carrier frequencies; a'motor and switching means driven thereby'forconnesting said tuning elements to said transmitter and receiver successively; a normally closed operating circuit for said motor; means including a selective circuit and switching means responsive to a modulating signal impressed on anyfsald carrier frequency incoming to said receiver," for deenergizing said motor when said receiver is tuned to said carrier; alternativemeans includ ing a switch and additional relay means for deenergizing said 'motor when said receiver is tuned to any said carrier frequency on which no carrier is incoming to said receiver; and means including an additional -switch for thereafter energizing saidtransmitter totransn'litoutgoing carrier on said frequency- 4. In a radio-telephone system: a subscriber's station including a radio transmitter and re ceiver' and a sulo'scribers telephoneset normally resting on a switch hook; a plurality oftuning elements for tuning said station to any of a pre selected number of carrier frequencies; means'in cluding said tuning elements, together with" a motor and switching means driven th'erebyfor tuning said station to said carrier frequencies successively; a normally clo'sed'opera'ting circuit for said motor; means including a selective'cir cuit and relay means connected thereto; res on; sive' to a modulating signal impressed on any" said carrier frequency incoming to said receiver for deenergizing said motor whensa'id" receiver'is tuned to said carrier; means incli'id'ing said switch hook and a carrier "current actuatedrelay in said receiver, for thereafter holding" said motor inoperative, upon removal of said. telephone set from said switch hook; means foririitiating an outgoingc'all from said station, in"- cludi'ng a manually actuated switch anda carrier current actuated relay for stopping; said motor when saidstatio'n is tuned to any said car rier frequency on which no carrieris incoming to said receiver; means including said carrierre layfor thereafter holding said motorinoperativeon receipt of incoming] carrier on said fr'e quency; and means including an additional switch for thereafter energizing said transmitter to transmit outgoing carrier on said' frequency.

5; In a radio-telephone system: a subscriber;- station including a radio transmitterand receiver and a. subscribers telephone 'set normally resting on a switch hook; a plurality of" tuning elements for tuning said station to any of a pre? selected number of carrier frequencies; means including said tuning elements, together with a motor and switching means driven" thereby for tuning said stationto said carrier frequencies successively; a normally closed operating circuit for said motor; means including a selectivecii; cult and relay means connected thereto, respon sl've to a-modulating signal impressed on any said carrier'frequency incomin'gt'o' saids'receivrfor'd energizing saidrnotor when. said receiver is tuned to'saidcarrier; a station'selector thereafter re spondingto coded interruptions of said modulat= ing signal for selecting and signaling saidvstae tion; means including said switch hook and-:a carrier current actuated relay in said receiver:

for: thereafter. holding, said; motor:v inoperative;

upon removal of said telephone set from said station including a radio transmitter and rei ceiver and a subscriber's telephone set normally resting on a switch hook; a plurality of tuning elements adapted for tuning said station to any of a, preselected number of carrier frequencies;

means including said tuning elements, together with a motor and switching means driven thereby for tuning said station to said carrier frequencies successively; a normally closed operating circuit for said motor; means including a selective circuit and relay means connected thereto, responsive to a modulating signal impressed on any said carrier frequency incoming to said receiver for deenergizing said motor when said receiver istuned to said carrier: a station selector thereafter responding to coded interruptions of said modulating signal for selecting and signaling said station; a circuit responsive to said station selection for thereafter holding said motor inoperative upon cessation of said modulating frequency; means including said switch hook and a carrier current actuated relay in said receiver, for thereafter holding said motor inoperative, upon removal of said telephone set from said switch hook; means for initiatin an outgoing call from said station, including a manually actuated switch and a carrier current actuated relay for stopping said motor when said station is tuned to any said carrier frequency on which no carrier is incoming to said receiver; means including said carrier relay for thereafter holding said motor inoperative on receipt of incoming carrier on said frequency; and means ineluding an additional switch for thereafter energizing said transmitter to transmit outgoing carrier on said frequency. a

7. In a radio-telephone system: a subscribers station including a radio transmitter and receiver and a subscriber's telephone set normally resting on a switch hook; a plurality of tuning elements for tuning said station to any of a preselected number of carrier frequencies; means including said tuning elements, together with a motor and switching means driven thereby for tuning said station to said carrier frequencies successively; a normally closed operating circuit for said motor; means including a selective circuit and relay means connected thereto, responsive to a modulating signal impressed on any said carrier frequency incoming to said receiver for deenergizing said motor when said receiver is tuned to said carrier; astation selector thereafter responding to coded interruptions of said modulating signal for selecting said station; means including a calling relayresponsive to said station selection for signaling said station and for thereafter holding said motor inoperative upon cessation of said modulating frequency; a locking circuit for said calling relay;, means including said switch hook and a carrierucurrent actuated relay in said receiver, for thereafter holding said motor inop- 14 erative upon'removal of said telephone set from said switch hook; means for initiating an outgoing call from said station, including a manually actuated switch and a carrier current actuated relay for stopping said motor when said station is tuned to any said carrier frequency on which no carrier is incoming to said receiver; means including said carrier relay for thereafter holding said motor inoperative on receipt of incoming carrier on said frequency; and means includingan additional switch and relay means for thereafter opening said calling relay locking circuit and for energizing said transmitter to transmit outgoing carrier on said frequency.

8. In a radio-telephone system: a subscriber's station including a radio transmitter and receiver and a subscribers telephone set normally resting on a switch hook; a plurality of tuning elements for tuning said station to any of a preselected number of carrier frequencies; means including said tuning elements, together with a motor and switching means driven thereby for tuning said station to said carrier frequencies successively; a, normally closed operating circuit for said motor; means including a selective circuit and relay means connected thereto, responsive to a modulating signal impressed on any said carrier frequency incoming to said receiver for deenergizing said motor when said receiver is tuned to said carrier; means including said switch hook for thereafter holding said motor inoperative, upon removal of said telephone set from said switch hook; means for initiating an outgoing call from said station, including said switch hook and a carrier current actuated relay in said receiver for stopping said motor when said station is tuned to any said carrier frequency on which no carrier is incoming to said receiver; additional means including said switch hook, but excluding said carrier relay, for thereafter holding said motor inoperative so long as said telephone set is off said switch hook; and means including a manually actuated switch for thereafter energizing said transmitter to transmit outgoing carrier.

9. In a radio-telephone system: a subscriber's station including a radio transmitter and receiver and a subscribers telephone set normally resting on a switch hook; a plurality of tuning elements for tuning said station to any of a preselected number of carrier frequencies; means including said tuning elements, together with a motor and switching means driven thereby for tuning said station to said carrier frequencies successively; a normally closed operating circuit for said motor; means including a selective circuit and relay means connected thereto, responsive to a modulating signal impressed on any said carrier frequency incoming to said receiver for deenergizing said motor when said receiver is tuned to said carrier; a station selector thereafter responding to coded interruptions of said modulating signal for selecting and signaling said station; means including a relay responsive to said station selection for thereafter holding said motor inoperative until said subscribers telephone set is removed from said switch hook; additional means including said switch hook for holding said motor inoperative upon removal of said telephone set from said switch hook; means for initiating an outgoing call from said station, including said switch hook and a carrier current actuated relay in said receiver for stopping said motor when said station is tuned to any said carrier frequency on which no carrier is incoming toxsaid receiver; means including said switch hook, but excluding said 

